In mechanical engineering, and most particularly in the field of aviation, it is often important to reduce the weight of parts, while maintaining satisfactory mechanical properties. By way of example, this is particularly important for the fan guide vanes of a turbofan engine, given that their dimensions can be as large as can the forces to which they are subjected. A turbofan engine has a fan that is normally situated at the front of the engine. It then delivers a stream of air that is split into a primary stream and an annular secondary stream around the primary stream. The primary stream is guided to a compressor, a combustion chamber, a turbine that drives the compressor and the fan, and is ejected together with the combustion gas via an exhaust nozzle in order to produce reaction thrust. The flow direction of the secondary stream is guided downstream from the fan, and the stream is ejected in substantially the same direction. Thus, thrust is due not only to the primary stream with the hot combustion gases, but also to the secondary stream of cool air driven by the fan.
The outlet guide vanes (OGV) of the fan are normally 30 to 50 in number and they have two functions in turbofan engines: structurally they form a connection between the fan casing and the remainder of the engine, and aerodynamically they straighten out the stream downstream from the rotary blades of the fan, thereby improving the thrust efficiency of the fan.
The term “bypass ratio” designates the ratio of the mass flow rate of cool air from the fan to the mass flow rate of hot combustion gas from the nozzle. Except at very high speeds, in particular supersonic speeds, the thrust efficiency of a turbofan engine increases with said bypass ratio. Increasing the bypass ratio thus constitutes an appropriate technique for reducing the energy consumption of turbofan engines, and consequently for reducing both operating costs and emissions of polluting and greenhouse effect gases. A high bypass ratio also presents other advantages, such as a lower sound impact.
Nevertheless, in order to increase the bypass ratio, it is normally necessary also to increase the diameter of the fan. However that presents several drawbacks, including in particular increasing both the size of the guide vanes and the forces to which they are subjected.
In the prior art, proposals have been made to limit the weight of fan guide vanes in various different ways, such as, for example: mechanically assembling guide vanes from a plurality of parts to form a hollow structure, or else forming a cavity in the vane by means of an insert that is dissolved away chemically. Nevertheless, those methods present drawbacks. Mechanically assembling a vane is lengthy and requires labor. In addition, it can give rise to unwanted deformations and stresses in the vane. Chemically dissolving an insert produces pollution and is therefore problematic from an environmental point of view.
In published patent application US 2007/065291 A1, proposals are made to produce rotary blades comprising a metal body that is machined with cavities that are filled with a material of lower density in order to reduce their weight. Nevertheless, machining such cavities is made increasingly expensive as their shape becomes more complex, thereby reducing the potential for reducing the weight of blades while maintaining mechanical qualities that are substantially uniform.